In respiratory care, it is often desirable to analyse and monitor the gas composition of a patient's exhaled and/or inhaled breathing gases. For instance, measurement of respiratory CO2, O2, N2O and anesthetic agents, such as halothane, isoflurane, enflurane, sevoflurane or desflurane, is useful in the care of critically ill patients undergoing anesthesia or mechanical ventilation. Typically, the gas concentrations of the patient's breathing gases are monitored by transferring a portion of the breathing gases through a sampling line to a suitable gas sensor or gas monitor.
The patient's exhaled breathing gases are usually saturated with moisture at body temperature. Thus, water naturally condenses when the gas sample is cooled to room temperature when passed through the sampling line. Collected condensate, together with secretion, bacteria or other contaminants possibly present in the breathing gases, may result in inaccurate readings at the sensor or even adversely affect a delicate gas monitor.
The accuracy of the gas concentrations obtained from a respiratory gas monitor also depends on the ability of the analyser system to direct the gas sample from the patient, through the tube of a sampling line to the gas sensor, without distorting the gas sample flow. One cause of distortion of the gas sample flow may be the adsorption on and/or absorption in the tube material of one of more of the components of the gas sample. Additionally, any physical obstacles in the gas sample line, such as valves or material seams, may distort the gas sample flow. Distortion of the gas sample flow, regardless of cause, can degrade rise time of the measured waveform making accurate analysis, especially at higher breath rates, difficult or impossible.
In order to protect a respiratory gas monitor from water and other contaminants, prior art gas sampling systems employs various means to separate liquids, bacteria etc. from the gas sample flow. It has for instance been known to include in the sampling line a water trap or another moisture separation means between the patient and the gas sensor. The challenge, however, is to design such a water trap or moisture separation means that achieves sufficient efficiency and capacity without distorting the gas sample flow.
U.S. Pat. No. 6,783,573 is directed to a gas sampling system for conducting respiratory gases. A gas sampling tube is configured to conduct the respiratory gases from a patient to a gas monitoring device. An output connector couples the gas sampling tube to the monitor. An output dryer tube is coupled between the gas sampling tube and the output connector. The output dryer tube is characterized by a tube length and a relative moisture removal efficiency. The relative moisture removal efficiency is dependent on the tube length. The tube length is selected to limit the moisture content of the respiratory gases being directed into the respiratory gas monitor to a predetermined level. The output dryer tube may be comprised of Nafion® or may be implemented using microporous filters or molecular sieves. An optional input dryer may be implemented using the same materials used to implement the output dryer.
WO 2005/072297 is directed to a liquid absorbing filter assembly and system using the same. It is disclosed a filter assembly for use in a sidestream gas sampling assembly. The filter assembly includes a hydrophilic liner lining the inner perimeter of a housing for wicking moisture from the gases to be monitored prior to the gases reaching a sensing mechanism. It is contemplated that the housing can be formed from an absorbent material or a gas drying material, such as Nafion®.
However, there is a need for improvement of prior art solutions for respiratory gas analysis in respect of, e.g., low distortion, long lasting moisture and/or water removal, or low cost.